The invention pertains to a process for preparing thermoplastic fibres by melt-spinning an alternating copolymer composed of alkenes and carbon monoxide.
Such a process is known from EP 310 171. This application describes the preparation of melt-spun fibres in a process where the polymer is spun at a temperature of at least Tm+20xc2x0 C., with Tm being the crystalline melting point of the polymer.
It was found that if this known process is used for fibre preparation on a larger scale, a number of problems occur which can be attributed in part to the thermal degradation of the polymer, which degradation occurs when the polymer is heated to a temperature above its melting point. These problems manifest themselves in an unstable spinning performance, with the risk of filamentation, discolouration of the polymer, a wide variation in the properties of the formed fibres or a deterioration of the mechanical properties of the formed fibres. These problems are objectionable when fibres are prepared on an industrial scale, i.e. in a continuous process where a large quantity of the desired product is made per unit of time.
Surprisingly, it has now been found that these problems do not occur when the polymer is spun at a temperature of at least TNF+5xc2x0 C., preferably TNF+10xc2x0 C., with TNF being the temperature at which the molten polymer is free of persistent crystallisation nuclei, which temperature can be determined with the aid of Differential Scanning Calorimetry, and where the residence time of the polymer at a single temperature or different temperatures above the melting point of the polymer satisfies:       [                  ∑                  n          =          1                ∞            ⁢                                    t            n                    ·          exp                ⁢                  xe2x80x83                ⁢                  (                      A            +                          B                              T                n                                              )                      ]    ≤  1
wherein tn is the residence time (in minutes) of the polymer at a temperature Tn (in K, with Tn greater than Tm, with Tm being the melting point of the polymer) and A and B are determined by measuring the viscosity of the polymer at different temperatures and residence times, as described hereinbelow.
In this application the term fibres refers to staple fibres as well as short fibres, filaments, and yarns (an assembly of filaments).
The term alternating co-polymers composed of alkenes and carbon monoxide in this application refers to polymers built up from alkene and carbon monoxide units in alternating sequence. This means that in the polymer chain each carbon monoxide unit will have two alkene units as its immediate neighbours, and vice versa.
In the process according to the invention, in the preparation of fibres with properties rendering them pre-eminently suitable for technical application, i.e. fibres of great strength and high modulus, preferably use is made of a polymer where 80-100% of the alkene units is composed of ethylene, more preferably a polymer where 80-100% of the alkene units is composed of ethylene and 20-0% of the alkene units is composed of propylene.
The intrinsic viscosity of the polymer employed generally is in the range of 0.3 to 2.5 dl/g, more particularly 0.5 to 1.90, and preferably 0.8 to 1.85 dl/g. The intrinsic viscosity of the polymer [xcex7] or LVN is the limiting viscosity number at an infinitesimally small concentration of the polymer in m-cresol, with       [    η    ]    =                    lim                  c          →          0                    ⁢              xe2x80x83            ⁢                        η          spec                /        c              =                  lim                  c          →          0                    ⁢              xe2x80x83            ⁢                                    (                          t              -                              t                0                                      )                    /                      t            0                          ·        c            
wherein t=time of outflow of the solution from the capillary, t0=time of outflow of the solvent from the same capillary, and c=concentration of the polymer in m-cresol in g/dl at 25xc2x0 C.
Such alternating copolymers are well-known. The preparation of these co-polymers is described, int. al., in EP 121965; EP 222454; EP 224304; EP 227135; EP 228733; EP 229408; EP 235865; EP 235866; EP 239145; EP 245893; EP 246674; EP 246683; EP 248483; EP 253416; EP 254343; EP 257663; EP 259914; EP 262745; EP 263564; EP 264159; EP 272728; and EP 277695.
In order to improve the polymer""s resistance to thermal degradation, adjuvants counteracting said degradation can be added to the polymer. Examples of such adjuvants are inorganic acid binding compounds such as calcium hydroxyapatite or alumina, polymer stabilisers such as sterically hindered phenols, carbodiimides, epoxy compounds, and phosphites, or combinations thereof.
In the process according to the invention the polymer is spun at a temperature of at least TNF+5xc2x0 C. It was found that when the polymer is not heated to TNF, there will still be nuclei in the (liquid) polymer, which on cooling of the polymer may cause very rapid crystallisation. In a spinning process this will lead to an irregular spinning picture, which will give rise to, int. al.
differences in diameter among filaments in a bundle which is spun in one go,
differences in diameter in the longitudinal direction of the filaments,
filamentation during spinning.
Such surface irregularities in the spun fibres will render them less suitable for use in most applications; generally speaking, it is great fibre regularity which is desired there. It has now been found that these irregularities do not occur when the polymer is spun at a temperature of at least TNF+5xc2x0 C.
Preferably, the polymer is spun at a temperature of at least TNF+10xc2x0 C., since at this higher temperature the spinning performance of the polymer will be improved further still.
In general, the polymer is subjected to a number of different treatments during the spinning process, which treatments are not necessarily carried out at the same temperature. Generally speaking, the time during which the polymer is subjected to such a treatment (the residence time) is not the same for every treatment step. For instance, the polymer may be melted and homogenised at a temperature T1 for t1 minutes and then conveyed through a heated pipe (T2, residence time t2), after which, via a spinning pump having a temperature T3 (residence time t3), it may be extruded through a spinneret plate having a temperature T4 (residence time t4).
In order to further reduce the polymer degradation, the process settings preferably are selected such that       [                  ∑                  n          =          1                ∞            ⁢                                    t            n                    ·          exp                ⁢                  xe2x80x83                ⁢                  (                      A            +                          B                              T                n                                              )                      ]    ≤  0.75
In melt-spinning alternating copolymers composed of alkenes and carbon monoxide use may be made of equipment also known to be used for melt-spinning other thermoplastic polymers. For instance, in the extrusion of the polymer use may be made of a spinneret plate such as is employed in melt-spinning other thermoplastic polymers, such as polyamide-6, polyamide-66, and polyester (polyethylene terephthalate). Such a spinneret plate has a number of capillaries having a diameter of 200 to 2000 xcexcm and an L/D ratio of 1 to 10.
Highly advantageous results are achieved when next to the spinneret plate a hot tube is mounted which has a temperature below the spinning temperature (Tspin). Preferably, a hot tube with a temperature between Tspinxe2x88x9250xc2x0 C. and Tspin is employed.
After spinning the resulting fibres may be wound or processed in some other way, e.g., to make non-wovens.
In order to obtain fibres with properties which render them pre-eminently suitable for technical application, the fibres need to be drawn. It is possible to draw the fibres immediately after they have been spun. Alternatively, the wound fibres can be drawn further in a separate process.
The resulting fibres are pre-eminently suitable for use as reinforcing yam in tyres on account of the favourable combination of high strength and high modulus, adhesion to rubber, and fatigue resistance.
Also, the fibres are highly suitable for reinforcing other rubber articles such as conveyor belts and vee belts. In addition, the fibres are highly suitable for use in technical fabrics, more particularly fabrics which exploit the fibres"" very good hydrolytic stability, e.g., fabrics used in paper making.
Measuring Methods
TNF 
The temperature at which the polymer is free of crystallisation nuclei (TNF) can be determined as follows:
3-4 mg of polymer are introduced into 10 xcexcl aluminium cups provided with a few holes in the lid. These cups are put into a Perkin Elmer DSC-7 Robotic system and subjected to the following temperature programme under a nitrogen atmosphere:
heating to Thold at a heating-up rate of 10xc2x0 C./min, with Tholdxe2x89xa7Tm(the crystalline melting point of the polymer),
keeping at a constant temperature of Thold for t minutes, and
cooling down to room temperature at a cooling rate of 10xc2x0 C./min,
with Thold being varied from Tm to Tm+50 and with there preferably being 1-3 minutes of keeping at a constant temperature.
The cooling curve enables the determination of both the peak temperature of the recrystallisation (Trc) and the onset of the recrystallisation (Trco). The value of Trc or Trco measured over one and the same period of keeping at a constant temperature is then plotted against Thold. On the axis on which Thold is plotted TNF can be read from the point of inflection in the curve found.
Determination of A and B
The parameters A and B of a polymer are determined as follows:
The polymer is melted in a Haake rheometer equipped with a device for determining the apparent melt viscosity of a polymeric melt by measuring the pressure drop in a 40 mm long capillary with a diameter of 2 mm. The rheometer should be set up in such a way that it is possible to keep the polymer at a temperature above Tm (the melting point of the polymer in question) for a particular time (ta). Next, the entrance pressure in the capillary is measured as a function of the temperature in the rheometer.
When the temperature increases, a lowering of the entrance pressure of the capillary is observed until a critical temperature Tc is reached. At this temperature a discontinuity in the pressure curve as a function of the temperature is observed.
The determination is carried out at least three times at different residence times ta. In this way three (or more) combinations of Tc/ta are produced.
Next, using linear regression, the connection is determined between 1/Tc (in K) as the x-value and ln(1/ta) (ta in minutes) as the y-value. The intercept of the line found=A, the coefficient of direction of the line found=B.
The invention will be further elucidated with reference to the following unlimitative examples.